Despite morphologic homogeneity, lymphocytes can be subclassified on the basis of immunologic capabilities. Recently, some aspects of the interaction of bone marrow and thymic dependent lymphocytes have been shown to be moderately resistant to the effects of ionizing radiation. In an investigation of this relationship, we have evidence to suggest a discrepancy in the radiosensitivity of these two populations of lymphocytes. The purpose of this project is to: 1) expand the observations on the relative radiosensitivity of defined subpopulations of lymphocytes to include: thymus versus bone marrow-derived; activated versus non- activated; primed versus non-primed. Cells are irradiated in vitro and cell viability evaluated in vivo and in vitro on the basis of a number of defined parameters. 2) define the physical-chemical basis of these differences in radiosensitivity and thereby gain an understanding of the pathogenesis of radiation-induced interphase death of lymphocytes. Defined populations are irradiated in vitro and enzyme activity in specific organelles correlated with ultrastructural alterations. Finally, the cytoplasmic membranes of defined subpopulations of lymphocytes exposed to varying amounts of radiation are examined for phospholipid spectra to include particularly the degree of unsaturation of substituted fatty acids (free radical interaction).